Figure 2.

The relationship between atropine concentration (the amount of atropine in a standard solution: given in nM) and [³H]QNB counts is linear from 1.95 nM to 25 nM. Generally, SAA is positive if [³H]QNB counts are under the level corresponding to 1.95 nM; therefore, SAA is considered positive [SAA(+)] when it is ≥1.95 nM (left red arrow); SAA is undetectable [SAA(−)] when it is <1.95 nM (left blue arrow). Nevertheless, because a small amount of SAA can cause AA in the central nervous system, clinicians should keep in mind that SAA might also be positive if [³H]QNB counts are above the level corresponding to 1.95 nM (0 nM < SAA < 1.95 nM; right yellow arrow). [³H]QNB: tritiated quinuclidinyl benzilate, SAA: serum anticholinergic activity.